Presently available AC to DC converters are based on a modular architecture, including an Input Network, a DC to DC converter and a load. The Input Network comprises a high frequency line filter and a slow bridge rectifier whilst the DC to DC converter constitutes the conversion stage of the AC to DC converter. The Input Network is continuously or discontinuously fed AC current from a main AC supply and is converted thereby to an appropriate DC output voltage which supplies power to the DC to DC converter stage. The conversion stage is designed to provide a high power factor rectified DC voltage from the main AC supply.
The benefits of high power factor include reduction in the RMS line current and in the line current harmonic distortions, so that the main AC supply may be more efficiently utilized, on the one hand, and, on the other hand, properly protected against the introduction thereto of harmonic distortions.
Such DC to DC converters are well known in the art and are described, for example, in U.S. Pat. No. 4,274,133 (Cuk et al); U.S. Pat. No. 4,355,352 (Bloom et al) and U.S. Pat. No. 5,416,387 (Cuk et al).
However, before discussing these references, it is instructive to make reference to an even earlier patent to Cuk et al namely U.S. Pat. No. 4,184,197 which issued on Jan. 15, 1980 and may be regarded as a seminal work in the field of DC to DC switching converters. To the extent that there is clearly overlap between the present invention and the above-mentioned U.S. patents and that a full understanding of the principles upon which the present invention is based may be derived from a close inspection of those earlier patents, all four of the above-mentioned U.S. patents are incorporated herein by reference.
U.S. Pat. No. 4,184,197 to which reference has been made discusses a large number of configurations employing high frequency transistor switches and having a nonpulsating input and output current. As explained in column 9 lines 62 ff of that patent, there are many applications of dc-to-dc switching converters wherein it is necessary to incorporate DC isolation between the input and output circuits of the converter. Cuk et al propose that this be achieved in such a manner as also to provide a capability for multiple outputs with different polarities and magnitudes, by employing an isolation transformer having a single primary winding and multiple secondaries. Such an embodiment is shown in FIG. 10 of U.S. Pat. No. 4,184,197 having a single transformer secondary winding for providing a single output only. As explained in column 10 lines 7 ff, the desired isolation is achieved by using two capacitances in place of the single capacitance used in non-isolating configurations such as shown in FIGS. 5(1) and 5(2) of the same patent. The provision of a one-to-one transformer couples the voltages across each of the capacitors so as to provide DC isolation. It is significant that Cuk et al present the circuit shown in FIG. 10 as providing a capability for multiple outputs with different polarities and magnitudes, notwithstanding the fact that FIG. 10 of the patent is clearly directed to a single output stage only.
It will be understood that, in order to extend the configuration shown in FIG. 10 to multiple output stages, the output capacitor must be duplicated in every channel. That is to say that for every secondary winding of the isolation transformer, there must be provided a corresponding output capacitor for storing charge when the so-called primary capacitance charges through the primary winding of the transformer. By such means, the desired power transfer between the input and output is achieved.
However, the output capacitor is typically at least 1000 .mu.F and is therefore a component of significant bulk. Thus, if such a capacitor needs to be connected in each output stage, then it is apparent that when multiple output stages are provided according to the configuration proposed by Cuk et al, then a power supply employing such a AC to DC converter and designed for multiple output stages will be unacceptably bulky.
AC to DC converters of the kind described form the major component in power supplies such as are commonly found in computers and the like. In view of the ever increasing requirement to decrease, as much as possible, the bulk of such computers it is clearly desirable to reduce the volume of the power supply provided therein. However, as is known, power supplies which are used in computers must be provided with multiple outputs in order to supply power to the different components therein requiring different supply voltages. Thus, the requirement to provide multiple output stages whilst, at the same time, reducing bulk to the minimum militates against the use of the circuit configuration proposed by Cuk et al in U.S. Pat. No. 4,184,197 because the provision of a separate output capacitor of significant bulk in each output stage necessarily increases the overall bulk of the resulting power supply.
U.S. Pat. No. 4,274,133 (Cuk et al) relates to an extension of the switching converter disclosed in the above-mentioned U.S. Pat. No. 4,184,197. The principal improvement in the later patent resides in the coupling of the inductors into a single magnetic circuit with two windings as described in the earlier patent. The coupling of inductances leads to at least a reduction to half of both current ripples. In order to achieve isolation between the input and output circuits, the energy transfer capacitance is divided into two parts and separated by an isolation transformer. FIG. 1 of the annexed drawings illustrates one possible realization of a switching converter having DC isolation Z between the input (source) circuit and multiple output (load) circuits as described in U.S. Pat. No. 4,274,133. In this case, an isolation transformer is provided and the transfer capacitance is split into a capacitor C.sub.a in the input circuit and a capacitor C.sub.b in the output circuit. In the case where multiple output circuits are provided, then a separate capacitor C.sub.b1, C.sub.b2 . . . C.sub.bn, is provided for each separate output. This is explained in column 4 lines 55 ff of the above-mentioned patent and is shown in FIG. 1 of the annexed drawings for a switching converter having two separate output circuits.
Finally, mention is made of U.S. Pat. No. 5,416,387 (Cuk et al) disclosing a single stage, high power factor, gas discharge lamp ballast based on the principles of the two earlier Cuk et al patents to which reference has already been made. Here again, 5the energy transfer capacitance is split so that a first component thereof is included in the input circuit whilst a second component is provided in the output circuit. The above-mentioned patent issued in May, 1995, i.e. some 15 years after the date of issue of U.S. Pat. No. 4,184,197 wherein a switching converter having multiple outputs is first disclosed. It is very clear that the provision of multiple outputs would completely militate against the requirements of a compact gas discharge lamp ballast which commonly suffer from high bulk owing to the large size and weight of low frequency magnetic ballast. It is apparent that providing multiple outputs in accordance with any of the Cuk et al configurations militates against the requirement for compactness owing to the increased bulk of the energy transfer capacitance which is duplicated in respect of each output circuit.
The above-mentioned U.S. Pat. No. 4,355,352 to Bloom et al attempts to reduces the large number of components in the earliest Cuk et al patent and does so by reducing the number of inductors. However, no reduction in the number of capacitors is proposed notwithstanding their significant bulk.
The above drawbacks notwithstanding, the Cuk et al configurations have become industry standards and it would clearly be desirable to provide a switching converter having all of the many advantages of the Cuk et al configurations, whilst being suitable for multiple output channels without requiring that the energy transfer capacitance be split in the manner shown in the above-discussed Cuk et al patents.